1. Field of the Invention
This invention relates to a method of imaging and an article prepared according to said method. More specifically, this invention involves an imaging method based upon (a) the conversion of a colorless or weakly colored (hereinafter referred to as "leuco") material to a highly colored derivative, (b) the conversion of highly colored material to a colorless or weakly colored derivative; or (c) the conversion of one colored material to another accompanied by a substantial shift in its absorption spectra. In this invention, color modification of a material is achieved by selective reaction of said material with an appropriate reagent across a divider which physically separates said material and said reagent.
2. Description of the Prior Art
The use of organic compounds as light sensitive media for photographic processes is well known, and is perhaps best exemplified by the diazo process. this process is based upon the photochemical changes effected in various diazonium salts upon exposure upon the capacity of these salts to form dyes. In general, dyes are formed when diazonium compounds or a diazotized amine couple with phenols or aromatic amines in an alkaline medium. Upon exposure to light, diazonium compounds undergo decomposition and are rendered incapable of coupling and thus dye formation. This type of reaction is commonly employed in copying papers. For example, in one such process a diazo-anhydride and a coupler are dispersed in a neutral medium, exposed to light of the appropriate wavelength and developed by contacting with ammonia fumes. In another process, paper coated with the diazo compound is developed by immersion in alkaline solution containing the coupler. In yet another system, the developing fluid (gaseous or liquid) and/or one or more of the color forming materials is isolated from the other essential components of the color forming process by microencapsulation. These capsules are pressure sensitive, and can be subsequently ruptured at the appropriate time during the image forming sequence and thus release the materials isolated therein which are necessary for image formation, U.S. Pat. No. 3,111,407.
The photodegradation products of diazonium compounds have also been employed as reducing agents in the conversion of the leuco form of phthalocyanine to a highly colored product, U.S. Pat. No. 2,884,326. In this system, the leuco form of phthalocyanine and the diazonium compound are dispersed in a water permeable colloid and the resulting dispersion coated on a supportive substrate. Upon drying of this dispersion, it is exposed to actiniic radiation and thereafter treated with a solvent capable of dissolving the leuco form of phthalocyanine or with a solution of a strong acid whereby a positive image, in the form of the colored phthalocyanine pigment, is produced in the coating.
In each of the systems described above, image formation is based upon the selective decomposition of a diazonium compound either to render it incapable of coupling with other materials or to assist in conversion of a colorless compound to a highly colored product.
Photodegradation has also been found useful in the preparation of relief images from photodegradable polymers, see for example, U.S. Pat. Nos. 3,558,311; and 2,892,712 (Examples VII and IX). In such processes, the selective illumination of an imaging layer containing such polymers results in degradation of the photodegradable polymer into lower molecular weight materials. The products of such degradation differ from the nondegraded polymer and therefore provide a basis for image formation. In the '311 patent the selective illumination of a polymer containing an oxime ester results in selective degradation of the polymer in the exposed regions of a film containing said polymer. The image is "developed" by removal of the degraded materials from the imaging layer by solvents which are specific for the degradation products, but unreactive toward the unexposed regions of the imaging layer. The '711 patent is similar in its disclosure, however, subsequent to imaging of the film of an unstabilized formaldehyde polymer, the image is "developed thermally". In both these patented imaging systems, the intensity and duration of exposure required to produce photolytic degradation within such films is quite extensive. Moreover, even after such extensive exposure, the image is still not visible, but requires development either with solvents or by thermal treatment. These systems are, thus, inefficient, expensive and generally impractical for use commercially.
Accordingly, it is the object of this invention to provide an imaging method wherein image formation involves the interaction of a highly colored, weakly colored or colorless material and at least one other reagent across a divider separating said material and said reactant(s).
More specifically, it is the principal object of this invention to selectively shift the absorption spectra of a highly colored, weakly colored or colorless material by interaction of said materials with one or more reactants across a divider in response to image information.
Another object of this invention is to provide a method for the selective modification of this divider so as to permit the desired interaction between the separated materials.
It is yet another object of this invention to provide a method for selective modification of the permeability of the divider to one or more of the separated materials by exposure of said divider to ultraviolet light.
Additional objects of this invention include the preparation of stable positive or negative images by the above method.